1. Field of the Invention
The invention relates to an improved method for coupling the position data from a ultrasonic magnetostrictive linear transducer to a programmable controller and for calibrating a linear transducer input module for use with transducers having different characteristics.
2. Description of the Prior Art
A typical programmable controller operates on a cycle where input data from input modules is coupled to a processor of the controller and is input by the processor into the user program, the user program is sequentially computed, and then output data are coupled from the processor module to the output modules. This input-compute-output process is continuously repeated. The elapsed time from input to output is usually several tens of milliseconds. Also, the processor sequentially reads the data from each input module and sequentially writes the data to each output module. The elapsed time to either read or write eight bits of input or output data from one input or to one output module is typically about 10 microseconds.
A typical ultrasonic magnetostrictive linear transducer has a stainless steel tube inside of which is a steel wire under tension. The transducer itself is mounted in a fixed location on a machine. When an electrical pulse of energy is applied to the wire, an electromagnetic wave is induced in the tube. A cylindrical magnet which is attached to a movable part of the machine encircles the transducer tube. When the wave enters the magnetic field caused by the encircling magnet, it creates a torsion wave which travels in both directions along the tube from the magnet location at a rate of approximately 9 micro-seconds per inch. At one end of the tube the wave is damped to prevent false waves from being detected At the other end the torsional wave is detected by a variety of methods to create a brief electrical pulse.
A typical electronic circuit arrangement for measuring distance will have an oscillator with an approximate 3.8 millisecond period to enable one to generate a brief transmit pulse, which initiates a torsional wave, to open a count gate for a counter chain which is counting pulses from a fixed-frequency oscillator, and to reset a predetermining transmit counter. When a pulse is received the predetermining transmit counter is incremented and another transmit pulse is sent immediately until the transmit counter has reached its predetermined count. When the predetermined count has been reached, the enabling signal for the fixed-frequency counter chain is removed, the count information is transferred to storage registers and the fixed frequency counters are reset. The storage registers contain the distance measurement.
In practice, no two magnetostrictive linear transducers have exactly the same torsional wave propagation time characteristics because of slightly different tube composition and other factors. The propagation time variation is approximately 0.7%. Because of the desire to maintain 0.001" accuracy and of the variation from one transducer to another, prior art measurement electronics had to match the frequency of the fixed-frequency counters to the individual transducer. In event of failure or damage to either the magnetostrictive linear transducer or the decoding electronics all components in such a system have to be replaced.
The matching of all components causes needless expense and complication for the user of this prior art scheme.
Even at the moderate update rate of 3.85 milli-seconds per linear measurement, multi-byte output data from the magnetostrictive linear transducer decoding circuitry can be easily updated so that, without synchronization, the data will change from the time that a programmable controller can store the first byte to the time when succeeding bytes can be read, resulting in an output error. As an example, suppose that the magnetostrictive linear transducer decoding data can vary from 0 to 48000 with a four-foot linear transducer and that the programmable controller reads first the low eight bits, then the next highest eight bits, then the highest eight bits Also suppose that when the first eight bits is read the data is 31999, and the processor correctly reads 99 but before the processor can read the second byte, the data changes to 32000. The processor will then read 20 and the data that is stored is the incorrect value of 32099. To overcome this problem, a synchronization interface from the magnetostrictive linear transducer decoder to the programmable controller input modules is either incorporated into the magnetostrictive linear transducer decoder or added as an external module. The synchronization interface is designed to receive a digital transition from a programmable controller output module at its input and then freeze the value of the magnetostrictive linear transducer decoder' s output data after a fixed amount of time, thereby enabling the programmable controller to read the correct data value.
One difficulty with this approach is the additional delay and programming overhead of having the programmable controller first output data and then read the input data. Another difficulty is that additional modules external to the programmable controller are required to convert the resolver shaft position to digital position and secondly, to synchronize the data to the programmable controller's input/output scan. In addition, the conventional input modules necessary to read the data into the programmable controller are, in themselves, expensive. Also, with rapidly changing input position data, from the time that an external synchronization circuit freezes the position data until such time that the programmable controller can input the data, the position may have changed so much that computed decisions based on the position data may have little value.